JPEG quality - from QT tables?

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blue-j
Posts: 68
Joined: 2007-06-12T14:03:18-07:00

JPEG quality - from QT tables?

Post by blue-j »

Is the JPEG quality reported by identify based on statistics of the quantization tables, or a metadata report? How reliable is it?

Thanks so much!
J
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glennrp
Posts: 1147
Joined: 2006-04-01T08:16:32-07:00
Location: Maryland 39.26.30N 76.16.01W

Re: JPEG quality - from QT tables?

Post by glennrp »

blue-j wrote:Is the JPEG quality reported by identify based on statistics of the quantization tables, or a metadata report? How reliable is it?
It computes a signature of the Q tables (sum and a hash) and compares that with the numbers from the tables used in IJG code.
See coders/jpeg.c, around lines 760-890 in the current version (6.8.8-7).
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